The present invention relates generally to heating/ventilation/air conditioning/and refrigeration (HVACandR) systems; it specifically addresses estimating the capacity and the coefficient of performance as well as defining and estimating an efficiency index and capacity index of a vapor compression cycle under actual operating conditions.
Air conditioners, refrigerators and heat pumps are all classified as HVACandR systems. The most common technology used in all these systems is the vapor compression cycle (often referred to as the refrigeration cycle). Four major components (compressor, condenser, expansion device, and evaporator) connected together via a conduit (preferably copper tubing) to form a closed loop system perform the primary functions which form the vapor compression cycle.
The efficiency of vapor compression cycles is traditionally described by a coefficient of performance (COP) or an energy efficiency ratio (EER). The COP is defined as the ratio of the heat absorption rate from the evaporator over the input power provided to the cycle, or conversely for heat pumps, the rate of heat rejection by the condenser over the input power provided to the cycle.
Knowing a vapor compression cycle""s COP is crucial to determine the electrical costs of operating the HVAC system over time. Faults, such as improper refrigerant level and dirty heat exchanger coils, may lower the efficiency of the HVAC system by lowering the capacity of the HVAC system or increasing the power consumption, or both. Thus, even if the instantaneous power consumption of the HVAC system does not vary, a lower capacity will demand longer run time from the system to remove the same amount of heat (in an AC or refrigeration system) from the conditioned space, thereby increasing the energy consumption over a period of time. Both effects of lowering capacity or increasing power translate into lower COP. Proper service of vapor compression cycle equipment is fundamental to keep the COP near the optimum values they had when they were manufactured.
The condenser and evaporator of vapor compression cycle equipment are heat exchangers. Capacity measurements of an HVAC system can be relatively complex; they require the knowledge of the mass flow rate and enthalpies in either side of the heat exchanger""s streams (refrigerant or secondary fluidxe2x80x94air or brinexe2x80x94side). To date, mass flow rate measurements in either side are either expensive or inaccurate. Moreover, capacity measurements and calculations are usually beyond the ability of a typical HVACR service technician.
Assessing the COP of vapor compression cycles is also challenging. The electrical power input and the unit capacity need to be simultaneously measured. Power measurements involve equipment that is expensive.
For air-cooled HVAC systems, the coefficient of performance depends strongly on the load under which the cycle is running. (In this description, xe2x80x9cair-cooledxe2x80x9d means that the condenser and evaporator are exposed to the atmosphere and all heat exchange takes place between the heat exchanger and air.) Thus, the COP of equipment running under different loads can not be directly compared. For that reason, an efficiency index (EI) and a capacity index (CI) are defined in the present invention to allow for comparisons between cycle performance in varying conditions.
The present invention includes a method for estimating the efficiency and the capacity of a refrigeration, air conditioning or heat pump system operating under field conditions by measuring four system parameters and calculating these performance parameters based on the measurements. In addition to the four measurements, the outdoor ambient temperature is used to calculate an efficiency index (EI), which is related to the COP, and a capacity index (CI). Power or mass flow rate measurements are not required in a primary embodiment of the present invention.
Once the EI and the CI of the system are determined, the principles and methods of the present invention can assist a service technician in locating specific problems. They can also be used to verify the effectiveness of any procedure performed by the service technician, which ultimately may lead to a more effective repair that increases the efficiency of the system. A procedure to estimate the operating costs of running the equipment, as detailed in the present invention, uses the values of EI and CI.
The present invention is intended for use with any manufacturer""s HVACandR equipment. The present invention, when implemented in hardware/firmware, is relatively inexpensive and does not strongly depend on the skill or abilities of a particular service technician. Therefore, uniformity of service can be achieved by utilizing the present invention, but more importantly the quality of the service received by the HVAC system is improved.
The present process includes the step of measuring liquid line pressure, suction line pressure, suction line temperature, and liquid line temperature. After these four measurements are taken, the suction dew point and discharge dew point temperatures from the suction line and liquid line pressures must be obtained. Next, the suction line superheat, the mass flow rate that corresponds to the compressor in the vapor compression cycle for the dew point temperatures and suction line superheat must be obtained, and the enthalpies at the suction line and at the inlet of the evaporator must be obtained. The capacity of the vapor compression cycle from the mass flow rate and the enthalpies across the evaporator can now be calculated.